The present invention relates to generally a cathode-ray tube and more particularly an electron gun for a color picture having improved resolution.
In general, the electron guns for cathode-ray tubes may be divided into the bipotential type and the unipotential type which are selected depending upon operating conditions and purposes of use. For instance, the bipotential type electron guns have been used in the picture or display tubes which must display the images with higher resolution because the bipotential type electron guns exhibit better convergence.
The bipotential electron guns comprise, in general, a cathode, a control grid electrode, an accelerating electrode and first and second cylindrical electrodes arranged in the order named. The thermal electrons emitted from the cathode are converged into the electron beam by means of a pre-focusing lens formed by the control grid electrode, the accelerating electrode and the first cylindrical electrode, and the electron beam is focused by the main electron lens formed by the first and second cylindrical lenses so as to strike against the phosphor screen of the cathode-ray tube. In the so-called pentagrid structure, an auxiliary focusing electrode is interposed between the control grid electrode and the accelerating electrode so that more better convergence may be obtained.
In order to minimize the diameter of the electron beam spot striking against the phosphor screen, the spherical aberrations of the electron lenses, especially of the main electron lens, must be minimized as practically as possible. One measure for minimizing the beam spot so far employed is to increase both the diameters of the first and second cylindrical electrodes. However, the increase in diameter of the first and second cylindrical lenses inevitably results in the increase of the neck portion of the cathode-ray tube in which the electron gun is disposed or enclosed, and the increase in diameter of the neck portion results in the inevitable decrease in deflection sensitivity.
In order to overcome the above problems, the cathode, the control grid electrode, the auxiliary focusing electrode, the accelerating electrode and the first cylindrical electrode are supported and electrically insulated from each other by electrically insulating rods in the neck portion in the prior art electron guns while the second cylindrical electrode is formed by the deposition of the film of, for instance, graphite, on the inside wall surface of the neck portion. Therefore, it is possible to enlarge the diameters of both the first and second cylindrical electrodes without increasing the diameter of the neck portion. However, the neck portion and hence the second cylindrical electrode has a low roundness. In addition, the correct coaxial alignment cannot be attained between the first cylindrical electrode which is elastically supported by springs in the neck portion and the second sylindrical electrode. As a result, even though the first and second electrodes are increased in diameter, the aberrations of the main electron lens cannot be minimized to a desired degree.